A Russian Su-27 fighter jet flew within five feet of a US Air Force RC-135 reconnaissance aircraft on Monday over the Baltic Sea, according to a US official, an encounter that was assessed to be "unsafe."

The Russian plane was armed and flew "erratically," the official added, noting that the maneuvers were deemed unsafe due in part to the proximity of the aircraft.

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Fox News was first to report the incident.

Meghan Henderson, a spokesperson for European Command (EUCOM), confirmed to CNN that an intercept took place while the US aircraft was "flying in international airspace over the Baltic Sea"

"Due to the high rate of closure speed and poor control of the (Russian) aircraft during the intercept, this interaction was determined to be unsafe," she said.

The intercept occurred amid rising tensions between the two nations in Europe and the Middle East.

On Monday, Russia warned that US aircraft operating in specific regions in Syria would be considered "air targets" for its forces after a US Navy jet shot down a pro-regime fighter jet.

While both the US and Russia say their forces are in Syria to fight ISIS, Russia has allied itself with the regime of Syrian President Bahsar al-Assad while the US-led coalition has aligned with groups that oppose both the Assad regime and ISIS.

Earlier this month, the US Air Force deployed long-range B-52 bombers and 800 airmen to the United Kingdom in support of joint exercises with NATO allies and partners across Europe.

Those exercises have primarily taken place in the Baltic Sea, the Arctic and along Russia's border with several NATO partners.

Over the weekend over 1,000 US and NATO troops conducted a defensive drill in the Suwalki Gap in the border area between Poland and Lithuania. That area is seen by most experts as a likely Russian target in the event of a NATO-Russia military confrontation.